Mafic lava, before cooling, has a low viscosity, in comparison to felsic lava, due to the lower silica content in mafic magma. Water and other volatiles can more easily and gradually escape from...
When the lava hardens, it becomes a rock. Igneous (definition: formed by solidification from a molten state) rocks are hardened lava/magma. Therefore, when lava hardens, it is an igneous rock.
It doesn't, necessarily. If it's made of a kind of mineral that is grey, then it will. Lava isn't inherently red... it's red because it's red hot. When it cools, the resulting rock will be whatever...